Safety device for series-connected incandescent electric lamps



Aug. 21, 1928. I I F. ECKHARDT ET AL SAFETY DEVICE FOR SERIES CONNECTED INCANDESGENT ELECTRIC LAMPS Filed July 16, 1926 Patented Aug. 21', 1928.

, UNITED STATES ranrz nckmnr AND 'MAX nonnaxnrrr, or 'nIsnNAcH, GERMANY.

surrey nnvrcn ron snares-corners]: mcnnnasonnr ELECTRIC LAMPS.

Application fllefiJuly 1.6, 1926, Serial No. 122,980, and in Germany January 29, 1926.

sistance of about the same size as that of the. filament which has ceased to function. Our invention conslsts in using tor establishing a parallel path, not only for thepar= ticular arrangement under discussion lit for .all kinds of parallel paths for the purpose of ensuring continuous operation, fine aluminium powder with loosely arranged particles separated by a thininsulating layer; g

The powder should not be as fine as possible, a powder of a flaky nature,-sim1- lar to graphite powder, being found preterable.

The aluminium powder may be used w1thout any addition, the thin layers of o nde of the powder granules themselves forming the insulating layer. It may also be mixed with non-conductors in powder form. The powder may be, used in the dry state or ma led with a sticky fluid or some other plasticislng agent, for instance insulating varnish, as long as the admixture does not dlsturb the close but light contact'between the separate anules coated with the thin insulating fryer.

Preferably the means for ensur ng the continuous operation according 'tothe present invention as is'well-known in the case of other types of safet within the lamps. T e material being used in the form of powder enables to be en- 4 closed very simply in a chamber in the foot of the lamp, such that the powder directly connects together the lead-in wires passmg through the chamber to the filament. Freterably the chamber should not be entirely .4 filled with. the powder, a small air filled space being left unoccupied. Dry, loose aluminium powder has been found particu-.

.larly suitable for use in this manner.

Owing to their constructional simpl1c1ty the aluminium powder resistances have been found particularly suitable for small lamps, even-for miniature lamps. According to the resent invention these safety devices are to lie used for Christmas tree-decorations and II for advertising letters and advertising ornar' devices, are housed ments in which it has hitherto not been possible to use such safety devices.

The accompanying drawing shows various constructional forms of the invention, Figs.- 1 and 1 being an actual constructional form of a lamp containing a safety device according to the present invention, Fig. 2 an arrangement for Christmas-trees the safety device being indicated diagrammatically and i Fig. 2 a-detail of Fig. 2.

In Fig. 1 of the drawing a is the glass bulb, in th neck of the bulb, c the so-called foot of the lamp, which supports the fusedin lead-in wires d. By a plug 7 of asbestos wool with a water-proof covering g, the foot of the lamp is closed to the outside. Dry aluminium powder has previously been introduced into the hollow space thus formed, such that, after the lamp hasbeen finished, a small air filled space! will remain.

Owing to its being in contact with the air, the aluminium powder becomes coated immediately withthe well-knownvery thin, but close layer of oxide, which presents sufficient resistance-tothe individual voltage of 14 volts. When however on the filament a failing, the lamps are subjected to the total voltage, the thin layers of oxide between the powderparticles are broken down. and the powder acts as a substitute resistance for the filament. Should further lamps fail and the current consequently rise to an unpermissible extent, the aluminium resistance will suddenly lose its conductivity and will act as a cut-out. 5

Should the oxide layer be destroyed durin" the transport of the lamps or owing to vi ration, it will be constantly formed again, as there is suflicient air in the space 71-, al though the plug f closes the'bulb in an airtight manner. The hollow space It also prevents the aluminiumv powder being pressed together excessively, when the plug) is inserted, and from being made conductive in consequence. On the other hand as is well known, the thickness of the layer of oxideon aluminium does not increase, so that the contact resistance always remains equal tothe ire-determined resistance, which is necessar for proper functioning of the device.

n Fig. 1 which only shows the small tube at the foot of the .lamp with the powder contained therein to a larger scale, it is assumedthat aluminium in powder form is used mixed with a non-conductive powder.

-The coarse granules 70 represent the metal granules, while the fine granules Z represent the non-metallic powder, for instance kaolin. 4 o

That the powder resistances .burn out when overloaded and that their resistance as a whole does not increase to such an extent that the whole series continues to burn with a reduced current-consumption, is probably accounted for by the fact that in the first place there is an increase inresistance between the separate granules which are in particularly slight contact with one another, while the value of the total resistance remains practically unchanged, whereupon the points of increased resistance become more heated and finally burnout, the air enclosed" in the chamber probably also playing a part.

In Fig. 2, four lamps of the lighting system of 'a Christmas-tree are shown, m are the usual fittings forsuch an arrangement, the clamping devices of which are indicated by clips at and are connected in series by means of flexible leads 0.

' connection in series, a hermetically sealed air-containing hollow space within the'"foot of, the lamp, lead-in wires for the filament extending through the said hollow space, a normally-non-conductive shunt to the filament constituted by a dry loose aluminium powder partially filling the said hollow space andhaving its granules in light contact with one another and with thin insulation between them and in electric contact with'the said lead-in wires, said shunt, on the filament being destroyed being capable of becoming conductive by the action of the total voltage applied to the series and of bridging over the faulty places.

2. In an incandescent electric lamp for connection in series a safety device comprising normally non-conductive shunt to the fila ent of'the lamp capable on the fila- The electrode wires d of the lamps have exposed parts al mcnt being destroyed of becoming conductive by the action of the total voltage applied to the series and when thus rendered conductive of bridging over the faulty place, the shunt being constituted by aluminium in powder form-having its granules in light contact and thin insulation separating the granules.

8. In an incandescent electric lamp for connection in series a safety device comprising a normally non-conductive shunt to'the filament of the lamp capable on the filament being destroyed of becoming conductive by the action of the total voltage applied to' the series and, when thus rendered conductive, of bridging over the faulty place, the shunt being constituted by aluminium in powder form having its granules in light contact and a thin insulating coating of aluminium oxide separating the granules.-

4. In an incandescent electric lamp for connection in series a safety device comprising a normally non-conductive shunt to the filament of the lamp capable on the filament being destroyed of becoming conductive by the action of the total voltage applied to the series and, whenthus rendered conductive, of bridging over the faulty place, the shunt being constituted by aluminium powder mixed with a plasticising agent and having its granules in light contact and'thin insulation separating the granules:

5. In a series of incandescent electric miniature lamps combined to form a Christmas tree illumination, a safety device for each lamp, this safety device comprising'a normally non-conductive shunt to the filament of the lamp, capable on the filament being destroyed, of becoming conductive by the total vo tage applied to the series and when thus rendered conductive of bridging *over the faulty place, the shunt being composed of aluminium powder and being con tained in the interior of the lamp.

In testimony whereof we have signed our names to this specification.

FRITZ ECKHARDT. MAX HOHNEKAMP. 

